I. Field of the Invention
This invention relates to digital pre-distortion of an inherently non-linear device. Specifically it deals with achieving a broadband digital signal with high resolution as a pre-distorted signal without the need for fast D/A converters using a high number of bits for high resolution.
II. Related Art and Other Considerations
One of the operator's main goals is to be able to offer high capacity to their customers in the network. High capacity in terms of number of channels in a cellular network requires in turn a tightening of the frequency plan. That is, more frequencies must be made available in a given area than before. The base station has to handle more carriers at the same site. Conventional systems like TDMA (DAMPS) and GSM require more channels and upcoming systems like the WCDMA instead requires a continuous wide bandwidth. This in turn calls for ultra linear amplifiers.
Linear amplifiers are used to amplify several carriers at the same time, as opposite to amplifying each carrier separately and then add them up in, for example, a hybrid-combiner. Hybrid-combiners such as 90° branch line couplers have the disadvantage that for each doublet of carriers there is a 3 dB power loss.
A linear power amplifier typically has an efficiency of about 6% but it keeps relatively constant efficiency as more carriers are added. Moreover, only one amplifier has to be used for all carriers. The main problem with power amplifiers is the linearity of the AM-AM characteristics, whereas hybrid combiners do not suffer from this. Most cellular systems require inter-modulation (IM) products to be in the order of 70 dB down from the carrier. Extensive work has been done to linearize power amplifiers of which feed-forward seems to be the most promising method. Inter-modulation products are simply subtracted at the output of the amplifier by comparing input and output signals of the main amplifier. An error-amplifier adjusts the level of the inter-modulation frequency products (output minus input).
Feed-forward can improve linearity to a certain degree but then it becomes very difficult to achieve the last few dB's necessary for full compliance. A way of further linearizing the amplifier is to pre-distort the input signal to the amplifier and compensate for the non-linearity. There are a number of ways as how to accomplish this. One way is to pre-distort within the feed-forward loop of the MCPA itself. Usually this is done in an analog RF fashion. RF pre-distortion (PD) may also be done outside the full MCPA.
Another way is to implement digital pre-distortion. Digital PD may be used whenever there is a digital combined signal at hand. The introduction of so-called software transceivers makes it particularly convenient to extract this signal. On a system-level there would be a digital software transceiver, a broadband digital-to-analog converter (DAC), some RF components and the RF MCPA basically connected to the antenna port. A digital pre-distorter would preferably be placed between the software transceiver and the DAC.
There is a need for pre-distortion as a means for or complementing classical linearization techniques for non-linear devices. Such devices may be single-carrier power amplifiers (SCPA) or for example multi-carrier power amplifiers (MCPA), or even passive devices.
Linearization, as it is usually implemented for broadband applications, is to use feed forward techniques. By using this technique it is possible to subtract unwanted signal components by comparing the signal before and after the non-linear device. Linearization can be accomplished to a certain degree but is normally implemented in analog fashion with its difficulties, which has to be taken care of, essentially at RF frequencies.
Digital pre-distortion, on the other hand, is usually accomplished at base-band or at some intermediate frequency (IF). Essentially the idea is to perform this where the best control of the signal is achieved, and also where the carrier frequency is much lower than at a real operating frequency.
Designers have been working with linearization basically from the point when amplification of electric signals started off. As a description of the State-of-the-art in linearization techniques one way is to implement pre-distortion in an analog fashion at analog RF directly in front of the non-linear device. Alternatively it may be incorporated within the feed-forward loop in the MCPA itself (if that is the non-linear device). Some ideas have been put forward to place the pre-distorter also at digital base-band as indicated in FIG. 7. The digital signal is copied and fed through a digital pre-distorter and then added to the original digital signal again before it is fed to the D/A converter. The correction is made completely to the digital signal.
For instance, in U.S. Pat. No. 5,598,436 is described a digital transmission system with pre-distortion. However, the circuitry uses different quantification levels for the phase and the amplitude, respectively.
Another U.S. Pat. No. 6,172,562 describes a pre-distorter for controlling the phase and amplitude in order to linearize a non-linear device. The document discusses the problem with high bandwidth demands when using high accuracy digital circuits. Separating the phase and amplitude corrections into two parallel branches then solves this problem.
In an article in Electronics Letters Vol. 33 Nov. 11, 1997 titled “Chip for wide-band digital pre-distortion RF power amplifier linearization” a custom chip for digital pre-distortion is disclosed in which the forward path and the adaptation/control path work with different speeds at a standard resolution of 14 bits. It is pointed out that the linearizer has to operate with a sampling frequency typically four to eight times higher than the signal bandwidth.
Thus, the drawback of today's solution is that a fairly large bandwidth has to be associated with the distorted signal. As most non-linear devices can be modeled as a power series (see FIG. 2), it is clear that signal components whose frequencies are linear combinations of the original ones will appear in the output signal. For example, if the main non-linearity is a x3-component, there will occur frequencies that occupy 3 times as large bandwidth as the original signal (FIG. 1 and FIG. 2). And likewise there will occur frequencies from non-linearity that have also an x5-component, which will give rise to actually 5 times as large bandwidth. The same argumentation can be used for further higher components.
As can be concluded from the above discussion, it is necessary to feed a signal into the device which in itself has the same required bandwidth as the distorted signal wanted to be improved. As there practically is a relation existing between the dynamic range (number of bits, or resolution) for a D/A converter and the sampling frequency, it is also clear that achieving a higher bandwidth of the pre-distorted signal also requires better D/A converters. A low resolution D/A converter may operate at a very high sampling rate, but oppositely it is difficult to design a high resolution D/A converter at the same high sampling rate.
Another view to the problem is that aliases (periodic copies) in the spectrum may occur if one is using a too low sampling rate. So, adding a pre-distorted signal to the original one will cause overlapping signal spectrum as indicated in FIG. 6. The original (analog) signal can no longer be uniquely filtered out properly without suffering from aliasing effects. Performing a D/A-conversion means essentially that the original spectrum in FIG. 3 should be possible to filter out in the digital representation as seen in FIG. 4. If the sampling rate from the start is too low in comparison to the signal bandwidth, aliasing effects as shown in FIG. 5 will occur.